FIELD OF THE INVENTION AND RELATED ART STATEMENT
The present invention relates to a video camera apparatus including a camera unit having a solid state image sensor, a video processor unit for generating a driving signal for the solid state image sensor and processing an output signal from the solid state image sensor to produce an image signal, and a cable for connecting said camera unit and video processor unit to each other. This invention also relates to a video endoscope apparatus comprising the above mentioned video camera apparatus.
As is well known in the art, the video endoscope apparatus comprises a flexible insertion section having a solid state image sensor such as a CCD (charge coupled device) arranged in a distal end thereof, a video processor unit for supplying a driving signal to a CCD and processing an output signal from a CCD to derive an image signal, and a signal cable for connecting the a CCD and video processor unit to each other, the signal cable having a length of several tens of centimeters to several meters. There are many types of endoscopes for respective applications and the lengths of the cables i.e. the insertion sections differ from each other. Under such a circumstance, it is not economical to provide a plurality of video processor units for respective endoscopes, so that it is desired to use a single video processor unit commonly for a plurality of endoscopes. When the cable has a long length, amplitude, D.C. level and phase of the driving signal which is supplied from the video processor unit to the CCD via the cable as well as amplitude and phase of the signal which is transmitted from the CCD to the video processor unit also via the cable vary for respective endoscopes, so that it becomes impossible to detect the image signal under the optimum conditions.
FIG. 1 is a schematic view illustrating the known video endoscope apparatus. In a distal end of an insertion section 601, there is arranged a camera unit 603 including a CCD 602. The camera unit 603 is coupled with a video processor unit 604 arranged outside the insertion section 601 by means of coaxial cable lines 605 and 606. The video processor unit 604 comprises a driving signal generator 607 for producing a driving signal for CCD 602, a double sampling circuit 608 for sampling an output signal from a CCD 602 to derive an image signal, and a synchronizing detection circuit 609 for deriving a color image signal. These circuits 607, 608 and 609 are operated in synchronism with each other. The driving signal supplied from the driving signal generator 607 via the cable line 605 contains horizontal transfer signal .phi..sub.H, charge reset signal .phi..sub.R, etc. In the present invention, among these signals, the transfer signal .phi..sub.H and reset signal .phi..sub.R are important, so that the explanation of the remaining signals is omitted. The output signal supplied from a CCD 602 to the video processor unit 604 via the cable line 606 has a waveform shown in FIG. 2A. This output signal contains a reset pulse portion S.sub.A which is produced by gating the output signal of a CCD, a feedthrough portion S.sub.B which has a level independent upon an amount of incident light, and an information signal portion S.sub.C whose level is changed in accordance with an amount of incident light as illustrated by a double-headed arrow. In the double sampling circuit 608, the feedthrough portion S.sub.B is sampled by means of a first sampling signal SHP-1 illustrated in FIG. 2B and the information signal portion S.sub.C is sampled with the aid of a second sampling signal SHP-2, and then a difference between sampled values is derived as an output image signal.
FIG. 3 is a circuit diagram showing the construction of the double sampling circuit 608. The circuit 608 comprises a first field effect transistor (FET) 608a which is driven by the first sampling signal SHP-1, a first capacitance 608b for holding a value sampled by the FET 608a, a second FET 608c driven by the second sampling signal SHP-2, a second capacitance 608d for holding a value sampled by FET 608c, and a differential amplifier 608e for deriving a difference between the values held by the capacitances 608b and 608d. By using the double sampling circuit having the above mentioned construction, it is possible to reduce or cancel 1/f noise etc. which might be produced by an amplifier for converting an amount of charges stored in a CCD into a voltage.
As explained above, in the known video endoscope apparatus, the first and second sampling signals SHP-1 and SHP-2 which define sampling instances are synchronized with the driving signal for a CCD 602, and this phase relationship is not changed even if the cable lines 605, 606 have different lengths. However, in practice, the phase of the signal supplied from the CCD 602 to the double sampling circuit 608 varies in accordance with the length of cables. For instance, when the cable has a longer length, a time delay of the signal becomes larger, so that the signal supplied from a CCD 602 changes from a waveform shown by a solid line in FIG. 4A to a waveform illustrated by a broken line. Then, the first sampling signal SHP-1 is generated at a timing outside the feedthrough portion S.sub.B and comes into the reset pulse portion S.sub.A, so that the value sampled by the first FET 608a no longer represents the level of the feedthrough portion S.sub.B. This results in that the output image signal from the differential amplifier 608e does not correspond to the amount of incident light and that the image reproduced by such an image signal does not have a high image quality.
In a color television camera apparatus in which a color mosaic filter is provided on the solid state image sensor, the color information is obtained in the form of a carrier wave, so that the color information is generally derived by means of the synchronizing detection. As stated above, when the phase of the output signal from a CCD 602 becomes uncertain, firstly the sampling timing in the double sampling circuit 608 becomes uncertain, and secondly the timing of the synchronizing detection for detecting the color information becomes uncertain. That is to say, the output signal of the double sampling circuit 608 contains a carrier including the luminance signal information and a carrier having the color signal information, and the synchronous detection circuit 609 detects the color information with the aid of a synchronous detection signal. This synchronous detection signal is supplied from the driving signal generator 607. As explained above, the first and second sampling signals SHP-1 and SHP-2 for the double sampling circuit 608 are synchronized with the driving signal, e.g. the reset pulse for driving the a CCD 602 and this phase relationship is determined suitable for an endoscope having a standard cable length. Therefore, when the length of cable is different from the standard cable length, the sampling timing and synchronous detection timing deviate from optimum timings.
Further, the cable has the electrostatic capacitance and D.C. resistance, so that when the length of the cable becomes longer, high frequency components of the reset pulse are lost as shown in FIG. 5B and a period t.sub.H during which the reset pulse has an effective high level becomes shortened and the duty cycle of the reset pulse changes. Moreover, due to the D.C. resistance of the cable and the input impedance of the a CCD, the pulse high level .phi..sub.RH is reduced to .phi..sub.RH -.DELTA.V and the pulse low level .phi..sub.RL is increased to .phi..sub.RL +.DELTA.V as depicted in FIG. 5C. This results in that the pulse peak value is reduced and the D.C. level of the pulse is increased. Therefore, the ideal driving signal shown in FIG. 5A is no longer supplied to the CCD, so that the CCD could not operate correctly and there might be produced errors in transferring and reading out charges.
Further, due to the frequency characteristic (response characteristic) of the reading out mechanism of the CCD and the electrostatic capacitance of the cable, the reset period tRA Can be obtained normally under the low intensity of incident light as shown in FIG. 6A, but when the incident light has a high intensity, the signal could not raise sharply and a reset period t.sub.RB becomes shorter than the normal value t.sub.RA as illustrated in FIG. 6B, so that the image signal could not be read out correctly.
In order to avoid the above mentioned drawbacks, in Japanese Patent Publication Kokai No. 60-80,429 and U.S. Pat. No. 4,539,586 there has been disclosed a video endoscope apparatus in which means for adjusting levels of signals supplied from the video processor unit to CCD and vise versa is provided within a connector for connecting the cable between the video endoscope and the video processor unit. However, this solution has another drawback in that the connector becomes complicated in construction and expensive in cost, so that the whole system becomes expensive. Moreover, the above mentioned publications do not suggest any measures for compensating the phase deviation due to the cable length variation.
In order to mitigate the above mentioned drawbacks, in the Japanese Patent Publication Kokai No. 62-82,782 there is described a television camera apparatus in which in addition to the cable for transmitting the driving signal to the CCD and the cable for supplying the image signal read out of the CCD to the video processor unit there is arranged a third cable for transmitting a driving pulse for reading the image signal from the CCD and the image signal read out of the CCD is sampled with the aid of the driving pulse. In Japanese Patent Publication Kokai No. 61-29,584, there is described the television camera apparatus comprising an additional control cable between the camera unit and the video processor unit and a current having a constant amplitude passes through the control cable and a voltage drop thereacross is detected. Then the detected voltage drop is compared with a reference voltage to detect a length of the cable and the decay of the high frequency components due to the cable is compensated for. Further, in Japanese Patent Publication Kokai No. 62-120,794, there is disclosed a television camera apparatus which includes a third control cable, a means for transmitting a monitor signal from the video processor unit to the camera unit, and a means for transmitting the output signal read out of the CCD together with the received monitor signal to the video processor unit. In the video processor unit, the transmitted and received monitor signal is extracted and its level is compared with a reference level, and a transmission distortion is compensated for in accordance with the output of the comparator. In these known television camera apparatuses, there is arranged the control cable in addition to the driving signal transmission cable and the image signal transmission cable, so that the construction of the cable system becomes complicated. Particularly, in the video endoscope apparatus, a diameter of the insertion section has to be made small as far as possible, so that it is practically difficult to provide the third cable within the insertion section.
In Japanese Patent Publication Kokai No. 61-187,470, there is described another television camera apparatus in which the reset pulse portion contained in the output signal supplied from the CCD to the video processor unit is extracted and a sampling pulse is generated by a phase locked oscillator which is phase-locked with the extracted reset pulse portion. However, in this television camera apparatus, it is possible to compensate the time delay introduced in the signal path from the CCD to a sample-hold circuit in the video processor unit, but it is impossible to compensate the decay in this signal path and the decay and delay introduced in the signal path from the video processor unit to the camera unit, so that it is practically difficult to obtain the image having the high quality.
In Japanese Patent Publication Kokai No. 61-147,680, there is described still another television camera apparatus in which the reset pulse contained in the output signal from the CCD is extracted in the video processor unit, its amplitude is compared with a reference value and an amplitude of the reset pulse contained in the driving signal for the CCD is adjusted in accordance with a result of the comparison. In this apparatus, the phase deviation due to the delay time introduced by the cable from the camera unit to the video processor unit could not be compensated for at all, so that the image signal could not be obtained correctly.
In Japanese Patent Publication Kokai No. 62-77,935, there is proposed a television camera apparatus in which the camera unit comprises a high frequency oscillator and an output high frequency signal therefrom is superimposed upon the output signal read out of the CCD, and in the video processor unit, the high frequency signal is extracted to detect the cable length in accordance with the detected level of the high frequency signal and the gain of the signal transmitted from the camera unit is controlled in accordance with the detected cable length. However, this television camera apparatus has a drawback that the high frequency oscillator has to be installed in the camera unit, so that the camera unit becomes complicated and large. This results in that the apparatus could not be applied to the video endoscope apparatus.